1) Field of the Invention
The present invention relates to methods of etching polyimide from a substrate and to the etched substrates prepared according to such methods. The methods of etching polyimide from a substrate and the resulting etched substrates are useful in the formation of hybrid circuits, printed circuit boards and tape automatic bonding structures.
2) State of the Art
Polyimides are polymers characterized by low dielectric constants and high temperature stability. Because of these and other characteristics, polyimides are frequently used in the electronics industry today. In particular, polyimides have found utility in printed circuit board technology. For instance, polyimides may be employed as a dielectric support in the manufacture of printed circuit boards.
Etching treatments for polyimides are well known. The etching treatments are important because, among other things, they are necessary for the formation of hybrid circuits, integrated circuits, tape automated bonding structures and the like. Nevertheless, there are difficulties associated with the present methods of etching polyimides.
One method employs the addition of solvents to etch polyimide from a substrate. However, conventional photoresists, which may be present in a substrate, may not always be strong enough to resist the polyimide etchant. In other words, the addition of solvents may also remove conventional photoresists and/or degrade the adhesion properties of conventional photoresists. The removal and/or degradation of photoresists makes the formation of circuits impossible.
Another method for etching polyimide involves potassium hydroxide. There are, however, three significant problems commonly associated with etching polyimide in potassium hydroxide. The first problem is long etch time. The long etch time results because most etching processes which use potassium hydroxide are relatively slow. The second problem associated with etching polyimide is poor sidewall definition. Poor sidewall definition is also referred to as poor etch factor. Finally, the third problem associated with etching polyimide in potassium hydroxide is the low loading factor of potassium hydroxide. That is, the etchability of potassium hydroxide decreases upon repeated or extended exposure to polyimide as a result of potassium hydroxide's limited capacity for polyimide residue.
In response to some of the difficulties, several methods were developed to address problems associated with etching polyimide from a substrate. For example, U.S. Pat. No. 3,791,848 to DeAngelo relates to an etching composition and a method utilizing such a composition for etching a surface of a polyimide. The method comprises exposing the polyimide surface to an etching composition comprising an aqueous solution of a basic compound and ethylenediamine. In particular, the ethylenediamine synergistically increases the etching rate of the polyimide.
U.S. Pat. No. 4,960,491 to Youlton relates to a process for etching a polymeric material by reaction with either an acid or alkaline etching solution. A reactive material, such as aluminum metal, is added to the etching solution in order to generate nascent hydrogen which promotes the etching process by increasing the etching rate and dispersion of the etch residue.
U.S. Pat. No. 4,846,929 to Bard et al relates to a process for etching fully cured or substantially fully cured polyimide which comprises contacting the polyimide layer with an aqueous solution of metal hydroxide followed by contact with an acid, followed by contact with a metal hydroxide solution. Etching of chemically cured polyimide can be enhanced by employing a presoaking in hot water.
U.S. Pat. No. 4,426,253 to Kreuz et al relates to etching the surface of polyimide material by the use of an aqueous solution of either ethyl or propyl alcohol, or a combination of ethyl and propyl alcohol, containing a basic compound. The aqueous solution contains 45 to 98% by weight of an alcohol selected from the group consisting of ethanol, n-propanol, isopropanol or a combination of these alcohols.